Optical communications which utilize a semiconductor laser as the light source have an advantage such that they enable long-distance transmission and large-capacity transmission. Especially, long-distance optical communications which utilize a long-wavelength semiconductor laser suitable for low-loss silica optical fibers have been widely used in trunk-line systems of communications, etc. from an early period. In general, in a transmission system for optical communications which employ optical fibers as the transmission path, a semiconductor laser is preferably used as the light source. In such a case, since, in the field of short-distance optical communications, only a relatively small light output is required for the light source to be used therein, a vertical cavity surface emitting semiconductor laser (VCSEL), which has advantages of small size and low power consumption, is applied as the light source for short-distance communications.
Digital signal transmission techniques in which short-distance optical communications that employ optical fibers as the transmission path are used are widely applied in the area of LAN or the like. In particular, their applications to interconnects in very high-speed computers have gained much attention in recent years. In the field of super high-speed computing systems, the continued development of semiconductor micro-processing technologies provides continuously rapid improvement in data processing speed of LSI. Further, an approach has been employed which utilizes parallel processing composed of multiple computing units to improve the overall data processing speed of the system. In a super high-speed computing system, data transfer is conducted between LSI chips mounted on the same board, between boards for mounting those LSI chips, and between housings (nodes) for accommodating multiple boards. The increased data processing speed of LSI itself causes rapid increase in the data traffic volume to be transferred between boards and between housings (nodes) for accommodating multiple boards. On such occasions, improvement in the data transfer speeds between boards and between housings (nodes) for accommodating multiple boards cannot catch up such rapid increases in the data traffic volume to be transferred, which becomes a bottleneck for the improvement in the overall performance of the system.
In the case when a digital signal is transmitted as an electric signal, the loss in a transmission path becomes larger as the frequency increases, and in the case of a high-bit-rate digital signal, distortion takes place in its pulse waveform. Due to this waveform distortion, in an electric signal transmission, the bit-rate at which data can be transmitted without bit errors is limited. Further, since crosstalk takes place between transmission lines which are arranged close to each other, it is difficult to arrange transmission lines at high density. On the other hand, in short-distance optical communications which use optical fibers as the transmission path, since crosstalk between two optical fibers will not take place, it is possible to arrange transmission paths at high density. In addition, the waveform distortion of an optical pulse signal due to phase dispersion caused by the optical fiber itself presents no problem in short-distance optical communications. Therefore, studies are underway on the application of optical interconnections, which enable a high-speed and high-density connection, to the data transfer between boards and between housings (nodes) for accommodating multiple boards.
As the light source to be utilized for optical interconnections, VCSEL which excels in high-density integration, power saving, and cost reduction abilities is viewed as a promising candidate. A VCSEL generally comprises a resonator including an emission region therein, which is sandwiched by semiconductor distributed Bragg reflectors (DBRs), and for high-speed modulation applications, generally a structure in which the optical length of the cavity is set to be one wavelength is used.
For example, Patent Document 1: JP 10-256665 A discloses a structure in which one-wavelength micro-cavity is employed as the resonator comprising an emission region therein, of which the upper and lower ends are sandwiched by semiconductor distributed Bragg reflectors (DBRs), the optical path lengths L1 and L2 from the quantum well layer, which works as the active layer, to the upper and lower ends of the resonator region are selected so as to satisfy the condition: L1=L2=λ0/2 respectively, as a result, the total optical path length: L=L1+L2=4. In that case, in the quantum well layer thereof, a high refraction-index layer which has a similar refraction index to that of a GaInNAs layer, which is a higher refraction index layer included in AlAs/GaInNAs composing the semiconductor distributed Bragg reflector (DBR), is used as a layer corresponding to the barrier layer/clad layer, which is formed on each side of a single well layer. Further, such a structure is selected in which an AlAs layer, which is the low refraction-index layer used to compose the semiconductor DBR, is in contact with each side of the quantum well layer.
Further, Patent Document 1 discloses and proposes a structure in which the optical length of the above described cavity is ½ wavelength, which is shorter than usual. This is a structure Which employs a half-wavelength micro-cavity as the resonator including an emission region therein, of which upper and lower ends are sandwiched by semiconductor distributed Bragg reflectors (DBRs), and in which structure the optical path lengths L1 and L2 from the quantum well layer, which works as the active layer, to the upper and lower ends of the resonator region are selected to satisfy L1=λ0/4 and L2=λ0/4 respectively so that the overall optical path length: L=L1+L2=λ0/2. In the structure, a layer of AlAs is used as the layer corresponding to the barrier layer/clad layer formed on each side of the quantum well layer (active layer), and it exhibits substantially the same refraction index as that of the AlAs layer which is the low refraction-index layer included in the AlAs/GaInNAs composing the semiconductor distributed Bragg reflector (DBR). Moreover, such a structure is employed in which a GaInNAs layer, which is the high refraction-index layer used to compose the semiconductor DBR, is in contact with each side of the micro-cavity.
Further, regarding the dielectric DBR, which is usable as the upper side DBR to form a VCSEL, a calculation method for estimating its effective optical path length, refraction index, internal loss, and the like has been publicly known (Non-Patent Document 1: Dubravko et al., “Analytic Expressions for the Reflection Delay, Penetration Depth, and Absorptance of Quarter-Wave Dielectric Mirrors”, J. Quant. Electr. vol. 28, no. 2, p. 514 (1992)).
Patent Document 1: JP 10-256665 A
Non-Patent Document 1: Dubravko et al., “Analytic Expressions for the Reflection Delay, Penetration Depth, and Absorptance of Quarter-Wave Dielectric Mirrors”, J. Quant. Electr. vol. 28, no. 2, p. 514 (1992)